The present invention relates to rotary fluid pressure units, including both pumps and motors, and more particularly, to such pumps and motors of the axial piston type.
In a typical axial piston pump or motor, there is a rotating cylinder barrel which includes a plurality of reciprocating pistons. The pistons engage a cam or swashplate, the position of which may be varied to adjust the displacement of the pump. The end of the cylinder barrel opposite the swashplate is seated against a valve plate which defines a fluid inlet and a fluid outlet. The inlet and outlet are connected, respectively, to the unit inlet port and the unit outlet port defined by the housing (which is sometimes referred to hereinafter as the "end cap").
Although the present invention may be used advantageously in an axial piston motor, it is especially advantageous when used in an axial piston pump, and, partly for ease of reference, will be described in connection therewith.
Axial piston pumps and motors have been widely used commercially for many years in a variety of industrial and mobile applications. One of the benefits of axial piston pumps is their "power density", i.e., the amount of hydraulic power output per unit volume of the pump. In spite of the inherently good power density of axial piston pumps, those skilled in the art continue to try to reduce the physical size of axial piston pumps, and further improve their power density. At the same time, there has been a trend in recent years to operate hydraulic circuits and components at higher and higher pressures, such that axial piston pumps are now routinely expected to be able to generate at least about 4000 to 5000 psi., without substantial degradation of performance or operating life.
However, one disadvantage of the effort to make axial piston pumps more compact, and operate them at higher pressures, is that various portions of the pump become sufficiently thin that, when subjected to such high pressure, those particular portions may deflect to such an extent as to lead to performance and/or durability problems within the pump.
In the conventional axial piston pump, the end cap defines a pair of kidneys, each of which is connected to a port, with a cored fluid passage interconnecting each kidney and port combination. In engagement with an interior surface of the end cap is the valve plate, fixed rotationally relative to the end cap. The end of the cylinder barrel is in sliding engagement with the valve surface of the valve plate as the cylinder barrel rotates. Typically, the valve surface of the valve plate which is in engagement with the cylinder barrel, is treated with a material such as a bronze alloy, to have a hardened surface operating against a hardened surface, and for the purpose of maintaining good wear characteristics between the valve plate and the rotating cylinder barrel.
In an effort to reduce the overall length of the pump, and also to facilitate a "tandem" arrangement of two pumps, it has become customary for the fluid passages in the end cap which interconnect the fluid kidneys and the ports, to be oriented radially relative to an axis of rotation of the pump. However, it has been determined in connection with the development of the subject embodiment of this invention that having at least a portion of the fluid passage oriented radially can result in a deflection or deformation of the portion of the end cap between the fluid passage and the surface adjacent the valve plate. Depending upon the particular configuration of the end cap, such deflection would normally manifest itself as a generally rounded, radially oriented, raised region. In one case, the width of the raised region, in a circumferential direction, was about the width of the adjacent fluid passage, and the resulting axial deformation of the end cap surface was in the range of about 0.000200 inches (0.00508 mm). Although the resulting deformation may seem relatively small, in absolute terms, it is common practice commercially to lap all of the engaging surfaces of the cylinder barrel, the valve plate, and the end cap to a flatness of about 0.000050 inches (0.00127 mm).
As is well known to those skilled in the art, it is intended that the end surface of the cylinder barrel ride on a hydrodynamic layer of oil as the cylinder barrel rotates on the valve plate. It will be understood by those skilled in the art that the term "oil" is used herein in the generic sense to mean and include any of the well know fluids typically used in such axial piston devices. It has been found that, if the interior surface of the end cap deflects, as described above, the result will be a corresponding deformation of the valve plate such that the oil film between the cylinder barrel and the valve plate will be "squeezed out" and metal-to-metal engagement will occur. There are two extremely undesirable results of this metal-to-metal engagement. One is a tendency for the lead in the bronze alloy coating on the valve plate to melt and be "sweated out" of the surface of the bronze coating. In the presence of the heat which is generated, the lead turns black and causes the valve surface of the valve plate to turn black, as though the surface had been "burned". This is primarily an appearance issue, but an important one with many customers.
Another problem is the potential for micro-welding, in which some of the bronze on the valve plate is transferred to the adjacent surface of the cylinder barrel, by means of the heat generated between the relatively rotating surfaces. Eventually, such micro-welding of the bronze destroys the surface finish of the parts, interfering with the generation of the hydrodynamic fluid film, and can destroy either or both of the engaging surfaces, thus substantially reducing the useful operating life of the pump.